Spread spectrum signals are widely used for the transmission of data when, for various reasons, the instantaneous power of the signal received is low, often below the thermal noise of the input stages of the receiver. Specifically, the spreadband transmission technique consists in transmitting information via an HF carrier modulated by a broadband signal. The band of the signal is then spread and occupies a frequency band equal to that of the modulating signal. This technique makes it possible to improve the resistance of the transmission to jamming by giving the signal transmitted a frequency spectrum akin to that of broadband noise. It makes it possible, moreover, by employing orthogonal codes for the modulating signal, to provide for the coexistence of several simultaneous transmission paths (code-based multiple access).
The spectrum spreading is usually obtained by phase-modulating the binary data to be transmitted or their transmission carrier by means of a pseudo-random binary string which plays the role of encryption code. Despreading at reception is done by correlating the signal received with a copy of the pseudo-random binary string that served for the spreading, which copy is produced locally in synchronism with that modulating the signal received. The correlation of despreading, over the duration of the pseudo-random binary string that served for the spreading or over a greater duration, less however than that of a datum transmitted enables the datum transmitted to be made to stand out from the ambient noise in which it was embedded.
For the transmission of radio signals in free space, use is made of a pair of transmit and receive antennas matched to one another in the sense that the receive antenna is designed to have maximum sensitivity with the waveform produced by the transmit antenna. The radio wave for a circularly polarized wave, has its electric field vector describing, in one or other direction, a circle (an ellipse) in a plane orthogonal to the direction of propagation defined by the straight line joining the pair of transmit and receive antennas. Two types of polarization are orthogonal if they correspond to waves whose electric field vectors are always orthogonal. When two waves have orthogonal polarizations, they are said to have cross-polarizations. A receive antenna optimized for a polarization exhibits minimum sensitivity for the cross-polarization. It is commonly accepted that the difference in sensitivity may reach 30 dB.
Between two mobiles or between a mobile and a fixed station, use is usually made of transmit and receive antennas corresponding to a radio wave with right or left circular polarization since such a radio wave has the advantage of being less sensitive to the propagation conditions which may introduce a rotation of the polarization plane and of being able to be picked up independently of variations in the attitude of the receive antenna with respect to the transmit antenna. In the case of right circular polarization, the cross-polarization is left circular polarization and vice versa.
One of the problems posed by the transmission of data by means of spread spectrum radio signals is the sensitivity of the receivers to nearby jammers, in particular in the case of transmissions of data with satellites, given the low power available onboard and the considerable distances separating transmitter and receiver. It is particularly acute in respect of satellite navigation systems where the receivers of navigation signals originating from satellites equip carriers deploying on the surface or in the vicinity of the surface of the globe in zones where the radio activity may be considerable.
A receiver of the satellite navigation system known by the name “Navstar Global Positioning System” (GPS) must be capable of utilizing a signal emitted by a satellite some twenty thousand kilometers away with a power of the order of some ten watts and reaching it with a power that is 20 dB below the ambient thermal noise. It is found at the output of the receive antenna with an input signal having a power of the order of −130 dBm that the gain of the despreading processing allows it to extract noise. However, the initial acquisition of synchronism between the pseudo-random binary string produced locally and that which modulated the signal emitted, which is necessary for the despreading operation, becomes difficult in the presence of a jamming signal having a power of more than 30 dB greater than that of the GPS signal, this representing a jamming signal power of the order of only −100 dBm which it is not difficult to exceed for an intentional or unintentional jammer placed at a distance from the receiver that is much smaller than the navigation satellites from which the useful signals originate.
Numerous procedures for decreasing the sensitivity of spreadband radio signals receivers to jamming have already been proposed. These known procedures implement three types of processing of the signal received: spectral or temporal filtering, spatial filtering and polarimetric filtering.
The antijamming techniques that call upon spectral or temporal filtering have been developed in order to escape from narrowband jamming occupying less than 10 to 20% of the frequency band of the spread spectrum useful signal. They consist in detecting through a spectral analysis the part of the useful band affected by the jamming and in eliminating it from the signal received through a bandstop filtering. Detection is based on the fact that the useful signal has, in the absence of narrowband jamming, a flat white noise spectrum in the useful band and that the presence of a narrowband jammer is manifested by a peak of greater or lesser width deforming the spectrum of the signal received. This bandstop filtering impairs the signal received since it deprives it of part of its content. It cannot therefore be used for broadband jammers. However, it makes it possible to escape from a certain number of garbling signals consisting of a fixed pure carrier termed CW (Continuous Wave), a sweep carrier termed SCW (Sweep Continuous Wave), pulsed carrier termed PCW (Pulsed Continuous Wave) or one modulated by a narrowband modulation signal.
The techniques of antijamming by spatial filtering consist in modeling the radiation pattern of the receive antenna in such a way as to favor reception in the directions of the useful signals to the detriment of reception in the directions of the jamming signals. They have the advantage of being effective for any type of jamming whatsoever, be it narrowband or otherwise. The modeling of the radiation pattern of the receive antenna may be static or dynamic.
An example of static modeling of the radiation pattern of the receive antenna is encountered in the receivers of the GPS satellite navigation system where use is made of receive antennas architectured in such a way as to exhibit a hemispherical radiation pattern, turned skywards and excluding the directions with low angle of elevation, thereby making it possible to favor the reception of signals originating from satellites over the reception of necessarily jamming signals originating from transmitters close to the surface of the ground.
Examples of dynamic modeling of the radiation pattern of the receive antenna are also encountered in satellite navigation system receivers for which the use has been proposed of receive antennas consisting of arrays of elementary antennas and of an adjustable spatial combiner producing an amplitude-weighted and phase-weighted sum of the signals picked up by the various elementary antennas, and an automaton for adjusting the spatial combiner of the antenna providing either for the formation of paths pointing toward the satellites emitting the useful signals, these paths corresponding to narrow beams directed toward the satellites concerned, which technique is known by the name “Beam steered antenna array”, or the creation of nulls in the reception pattern of the global antenna, in the directions of the jammers, which technique is known by the name “Controlled Reception Pattern Array” or, CRPA for short.
The “beam steered antenna array” technique is based on a priori knowledge of the relative positions of these transmitter satellites with respect to the receiver.
The CPRA technique is based on:                static forming of several independent reception paths on the basis of the signals picked up by these elementary antennas of the array antenna used for reception, independence between reception paths signifying that none of them reduces to a simple linear combination of the others,        generally, the central antenna is chosen as main reception path, the others being regarded as auxiliary, and        dynamic forming of a so-called “degarbled” reception path by supplementing the signal of the main reception path with a linear combination of the signals of the auxiliary reception paths, amplitude-weighted and phase-weighted with the aid of coefficients that are altered dynamically so that the signals of the so-called “degarbled” reception path and of the auxiliary reception paths are decorrelated.        
It is shown that the possible number of independent reception paths is less than or equal to that of elementary antennas of the receive array antenna and that, to be able to eliminate N jammers, there must be at least N independent auxiliary reception paths and hence an array antenna comprising at least N+1 elementary antennas must be available at reception.
The technique of antijamming by polarimetric filtering stems from the radar sector. It is based on the assumption according to which the signal picked up at reception, under cross polarimetry is essentially due to the jamming, and that it is thus possible by modifying the phase and the amplitude of the signal received under cross-polarimetry, to produce an image of the jamming affecting the signal picked up under matched polarization with a view to deducting it from the signal picked up under matched polarization and thus improve the signal-to-noise ratio of the useful signal. This assumption is strengthened furthermore in the sector of receivers for satellite navigation systems, by the fact that, for a receive antenna with hemispherical radiation pattern, it is impossible to obtain a matched polarization path or a cross-polarization path in all the directions of its radiation pattern, matching the polarization of the wave received in the matched polarization path or the crossing of polarization with respect to the wave received in the cross-polarization path being complied with well around the normal to the hemisphere and being complied with less and less on approaching the edges of the hemisphere. This impossibility further strengthens the predominant presence, in a cross-polarization path, of the most troublesome jamming signals, those with the same polarization as the useful signal, since they originate at the receive antenna via smaller angles of incidence than those via which the useful signals arrive, smaller angles of incidence for which the cross path is less mismatched.
As shown by American patent U.S. Pat. No. 3,883,872, thought has quickly been given to the application of this technique of antijamming by polarimetric filtering to the reception of an information channel in multichannel communication systems employing the same frequency band to transmit two simultaneous information channels according to orthogonal polarizations, the second information channel transmitted under cross-polarization then being regarded as an undesirable jamming signal.
More recently, it has proposed that this technique of antijamming by polarimetric filter be implemented within the framework of a GPS receiver. The GPS receiver is then equipped with a receive antenna simultaneously delivering a matched polarization path and a cross-polarization path. The signal of the cross-polarization path of the receive antenna is subtracted from the signal of the matched polarization path, while altering its amplitude and its phase to obtain an amplitude minimum for the resulting signal, for example by minimizing the output power under constraint. For further details regarding this technique of antigarbling by polarimetric filtering, reference may be made to American patent U.S. Pat. No. 5,712,641.
All these antijamming techniques improve the insensitivity of receivers of spread spectrum radio signals to jamming without however achieving total insensitivity, so much so that improvements are still sought. Thus it has been proposed, especially in American patent U.S. Pat. No. 6,141,371, that the techniques of antijamming by temporal filtering and by spatial filtering more precisely of CRPA type be combined.